The present invention relates generally to crystalline materials. More particularly, the present invention provides an ultra-low defect density, large-area bulk gallium nitride wafer which is free of bowing. The resulting wafer can be used, for example, as a substrate or seed crystal for subsequent growth of crystalline materials, e.g., GaN, AlN, InN, among other applications. The present invention provides a method using a first seed structure which is large-area and free of bowing or other imperfections. The present invention includes lateral growth in one or more embodiments. In a specific embodiment, the resulting large-area bulk gallium nitride wafer can be used in an ammonothermal growth process or the like. In a specific embodiment, the resulting bulk gallium nitride wafer can be used to produce a volume of gallium nitride material which can be cut into a plurality of bulk gallium nitride wafers. Merely by way of example, the present wafer can be used in applications such as light emitting diodes, laser diodes, integrated circuits, MEMS, medical devices, combination of these, among others.
Single-crystal gallium nitride (GaN) containing compounds and related alloy compounds containing aluminum and indium (AlN, AlxGa1-xN, InN, InxGa1-xN) and possibly boron are useful semiconducting materials. Such semiconductor materials can be useful for a variety of applications due to their large bandgap and high chemical and thermal stability. In recent years, there has been significant technological advancement in electronic and optoelectronic devices based on these materials, such as transistors, solar cells, light-emitting diodes, and lasers, among others. Although some of these products are available in the commercial market today, lack of a suitable GaN substrate on which to grow these materials remains a limitation to both performance and providing low cost, volume production of devices.
Conventional approaches to growth of GaN, AlN or InN containing compounds (collectively referred to herein as “(Al,In)GaN” compounds or “(Al,B,In,Ga)N” compounds) and devices employ foreign substrate materials (where “foreign” herein refers to a material containing one or more primary chemical species which is different from Ga, Al, In, or N), a process known as “heteroepitaxy”. Heteroepitaxial approaches to growth of (Al,In)GaN containing compounds result in epitaxial films with high defect densities (commonly 1010 cm-2 or higher) due to the large lattice mismatch, chemical dissimilarity and thermal expansion coefficient difference between the nitride materials and substrate. The presence of defects is well-known to be detrimental to device performance. The thermal expansion coefficient difference between the substrate and the epitaxial layer in heteroepitaxy results in strain gradients in the material which can lead to wafer curvature, referred to as bow or warp, after growth. As used herein, the terms bow and warp are used in a manner which is well understood in this art. Definitions, for example, can be found from SEMI (www.semi.org), but others can be commonly known.
Additionally, heteroepitaxy of gallium nitride and related materials on foreign substrates can result in incorporation of impurities into the gallium nitride material. This can be caused by the exposed foreign substrate material being introduced into the growth environment. By varying techniques, the growth environment can involve corrosive gases, high temperatures and pressures, among other process characteristics. Such process characteristics can cause decomposition of the foreign substrate material into its constituent elements, or diffusion of one or more constituent elements into a growing material, or a combination of these effects. For example, common substrates for nitride epitaxy such as sapphire (Al2O3) and SiC can decompose into their constituent elements Al and O, or Si and C, respectively. These constituent elements can then incorporate into growing nitride materials as impurities and can act as dopants, causing what is known as “unintentional doping” or “UID”. There is therefore a need for bulk GaN substrates of high crystalline quality, ideally cut from large volume bulk GaN ingots.
Ammonothermal growth is a promising low cost and potentially highly scalable approach to produce such a GaN ingot. Ammonothermal growth has provided high quality crystalline material, however, drawbacks exist. As an example, ammonothermal growth techniques currently lead to small sized crystals, which are often not useful for commercial applications. Additionally, a significant limitation is that defects in the seed material used for ammonothermal growth are known to replicate on any grown crystal structures. For example, a commonly used method to produce bulk or pseudo-bulk GaN layers is hydride vapor phase epitaxy (also generally known as HVPE). The high growth rates achievable by HVPE allow heteroepitaxy of thick films (on the order of hundreds of microns) on a foreign substrate, resulting in dislocation densities on the order of 107 cm−2, for example. GaN layers can be sliced from such HVPE material and subsequently used as a seed for ammonothermal growth, however threading dislocations, strain, and grain or tilt boundaries exist in the HVPE material and will persist in any epitaxial layers subsequently deposited and in any devices fabricated from these layers. These and other limitations often exist with ammonothermal techniques.
From the above, it is seen that techniques for improving crystal growth are highly desired.